Superconductor materials have long been known and understood by the technical community. Low-temperature (low-Tc) superconductors exhibiting superconductive properties at temperatures requiring use of liquid helium (4.2° K), have been known since about 1911. However, it was not until somewhat recently that oxide-based high-temperature (high-Tc) superconductors have been discovered. Around 1986, a first high-temperature superconductor (HTS), having superconductive properties at a temperature above that of liquid nitrogen (77° K) was discovered, namely YBa2Cu3O7−x (YBCO), followed by development of additional materials over the past 15 years, including Bi2Sr2Ca2Cu3O10+y (BSCCO), and others. The development of high-Tc superconductors has brought potentially, economically feasible development of superconductors utilizing liquid nitrogen, rather than the comparatively more expensive cryogenic infrastructure based on liquid helium.
A great deal of interest has been generated in the so-called second-generation HTS conductors that have superior commercial viability. These conductors typically rely on a layered structure, generally including a flexible substrate that provides mechanical support, at least one buffer layer overlying the substrate, the buffer layer optionally containing multiple films, an HTS layer overlying the buffer film, and an electrical stabilizer layer overlying the superconductor layer, typically formed of at least a noble metal.
Commercial availability and consistent, gradual improvements in second-generation superconducting conductors opens up new application areas for HTS conductors. One such promising application involves high-field magnets operating at above-helium, and potentially, at a liquid nitrogen temperature. Effective prevention of quenches in these devices is an important reliability factor. Unlike low-Tc superconductors, the HTS conductor is less prone to quenching. This is primarily due to a combination of much higher specific heat of the HTS conductor at operating conditions, and a less steep I-V characteristic (lower n-value) compared to the low-Tc counterparts. At the same time, whenever quench in HTS conductors occurs, its detection and management presents a serious engineering problem. The same factors that suppress quench occurrence result in very slow development of thermal instability and inhibiting of its propagation along the HTS conductor. Normal zone propagation velocity reported for a practical second generation superconductor is in the range of 0.1-1 cm/s, which is 103-104 times less than in a low-Tc superconductor. Slow developing thermal instability means that initially a small local hot-spot (or quench region) is formed in the wire, and significant heating can occur there prior to the surrounding region quenching to the normal resistive state of the conductor. This leads to a quick local degradation of the YBCO material due to oxygen loss or due to complete conductor burn-out. At the same time, voltage associated with formation of one or more localized hot-spots is always small, proportional to the hot-spot dimensions, which makes it hard to detect the quench signal in the background of voltage noise.
A number of active and passive approaches to the quench detection problem have been proposed. They include active solutions, such as individual voltage monitoring in magnet sub-sections (see, e.g., J. H. Schultz, “Superconducting Magnets, Quench Protection”, Wiley Encyclopedia of Electrical and Electronics Engineering, pp. 1-27 (1999); and B. Seeber, Handbook of Applied Superconductivity, pp. 542-543 (1998)) and acoustic noise detection (see, e.g., Trillaud et al., “Protection and Quench Detection of YBCO Coils. Results with Small Test Coil Assemblies”, IEEE Trans. Appl. Superconductivity, Vol. 17, No. 2, pp. 2450-2453 (June 2007)) and passive ones, such as the use of a material with high-heat capacity (diamond, sapphire) or a switching dielectric-conductor material (ZnO) as a surrounding shell between the neighboring turns of the magnet wire (see, e.g., Oberley et al., “Improved Dielectric Materials for Passive Quench of High Temperature Superconductors”, presented at the International Cryogenic Materials Conference, Keystone, Colo., Paper No. M2-L-04, (Aug. 29-Sep. 2, 2005)). Also, complex active detection techniques based on 3-D computer modeling of the thermal response are being developed (see, e.g., Bai et al., “Quench Propagation Properties Analysis of High-Temperature Superconductors Using Finite Element Method”, Physica C, 436, pp. 99-102 (2006)).
Notwithstanding the above, a need continues to exist in the superconducting art, and in particular, in the art of second generation HTS conductors, for provision of commercially viable conductors, methods of fabrication, and articles utilizing the same which incorporate an early quench detection facility.